1. Field of the Invention
The present invention relates to digital image processing, and more particularly to a method and apparatus for improving the contrast of a digital image by a content-adaptive tone mapping function.
2. Description of the Prior Art
Digital image tone mapping is a function typically supported in the video processing unit of a digital image rendering device, such as a display device or a printer. The tone mapping function converts luminance code values of an input image from a predetermined range defined by the bit resolution of the input image to another desired range associated with the rendering device. The input image may be either a monochrome image or a color image. For the monochrome case, the gray levels of pixels are themselves used as the luminance components. For the color case, each pixel of the input image typically contains three or four components to define a color. In RGB color space, for example, these primary colors are red, green and blue, and the luminance component of each pixel can be obtained by a weighted summing of the red, green, and blue components of the pixel data.
The tone mapping function may be static or adaptive A static tone mapping function is defined by an invariant transfer function for all input images. An adaptive tone mapping function, on the other hand, may be dynamically adjusted according to desirable characteristics of the input image. Contrast improvement is usually taken into account when attempting to generate a rendered image that looks right with respect to the original scene. Due to the wide range of distributing status of pixels in different images, it is typically difficult to use a single static tone mapping function to fit all kind of images for contrast improvement purpose.
As a result, there exists a substantial need for a method and apparatus involving an adaptive tone mapping function capable of improving image contrast, and preferably simple, low cost and easy to implement.